1. Field of the Invention
The present invention relates to a transmission type flat CRT (cathode ray tube) and, more particularly, to a flat CRT having a carbon layer on an inner surface of a back panel.
2. Description of the Prior Art
A conventional reflection type flat CRT (cathode ray tube) 6 includes a flat glass tube envelope 5 that is formed by a triple structure of a screen panel 1, a front panel 2 and a funnel portion 4 having a neck portion 3 as shown in FIG. 1. More specifically, the flat glass tube envelope 5 consists of the screen panel 1 and the front panel 2 that are bonded in an opposing relation by frit glass so as to form a flat space 7 therebetween, the funnel portion 4 being bonded and sealed to one side of the screen panel 1 and the front panel 2 similarly by frit glass and the neck portion 3 being welded to the funnel portion 4 at its end towards the small-diameter opening. An electron gun 8 is disposed within the neck portion 3.
A phosphor screen 9 is deposited on the inner surface of the screen panel 1 via a reflecting layer (not shown) formed of aluminum vapor deposition film or the like. An electron beam e emitted from the electron gun 8 impinges upon and scans the phosphor screen 9 in the horizontal and vertical directions. Then, an optical image excited and made luminous by the electron beam e is observed from the front panel 2 side opposing the screen panel 1.
There is developed a transmission type flat CRT 16 using a flat glass tube envelope 15 that is formed of a triple structure composed of a screen panel 11, a back panel 12 and a funnel portion 14 having a neck portion 13 as shown in FIG. 2. A phosphor screen 19 is formed on the inner surface of the screen panel 11 through a transparent conductive film, e.g., ITO (indium oxide-tin oxide) film (not shown). Then, an electron beam e emitted from an electron gun 18 impinges upon and scans the phosphor screen 19 in the horizontal and vertical directions and an optical image excited and made luminous by the electron beam e is observed from the screen panel 11 side.
In such transmission type flat CRT 16, when a part of the electron beam e strikes the back panel 12 to place negative electric charges 20 on the inner surface of the back panel 12, there is then the risk that a passage of the electron beam e is displaced by the negative electric charges 20. In particular, the influence of negative electric charges 20 on the electron beam e becomes pronounced in the top of the CRT 16 so that a pseudo signal occurs. The pseudo signal is an aliasing noise or smear noise which occurs in an image when the electron beam e scans the phosphor screen 19 on a scanning line inside the original scanning line due to the negative electric charges 20 thus charged as shown by a dashed line in FIG. 2.